Silicon on insulator (SOI) waters are widely used for the purpose of reducing parasitic capacitance, thereby accelerating device performance. Attractive among SOI wafers are silicon on quartz (SOQ) and silicon on sapphire (SOS) in which the handle wafer is a transparent insulating wafer. SOQ is expected to find use in optoelectronic applications that take advantage of high transparency of quartz, or in high frequency devices that take advantage of a low dielectric loss of quartz. Since SOS whose handle substrate is made of sapphire has high transparency, a low dielectric loss, and a high thermal conductivity, which is unavailable from quartz. SOS is expected to find use in heat-generating high frequency devices.
For stacking of a single crystal layer of quality, it is ideal to form a silicon thin film by the bonding/transfer process from a bulk silicon wafer. While several processes including hetero-epitaxial growth of a silicon layer on R-face sapphire, and CG silicon process involving growth of non-single-crystal silicon on glass and subsequent laser annealing to improve crystallinity have been developed, allegedly none of them surpass the bonding process.
It is noted that bonded wafers of heterogeneous materials (e.g., SOQ, SOS and SOG) leave concerns about defects induced during the process. Specifically, typical device processes involve heat treatment at a high temperature in excess of 850° C. to form a gate oxide film. In these bonded substrates of heterogeneous materials, hereinafter referred to as “heterogeneous composite substrates,” such high-temperature process applies strong compressive/tensile stresses to the silicon thin film to form various defects such as micro-cracks. The cause originating from the heterogeneous composite substrate is a substantial difference in coefficient of expansion between the support substrate known as “handle substrate” and the upper layer or silicon thin film. This is the essential problem of heterogeneous composite substrates.